A variety of optical lenses has been conventionally assembled and used in different types of optical devices, information processors, and so on. The most familiar of the lenses has the symmetrical aspheric or nonspherical surface, which is made by polishing or lapping the surfaces of the cut or molded glass blank with the use of lapping of honing machines to the final form. Many types of the specific lapping machine have been heretofore developed to generate the symmetrical aspheric surface on the lens. Most prior lapping machines, nevertheless, are envisaged to remove any rough surfaces left on the lens to improve the surface finish. The lapping of the workpiece is a slow process because the slow rotating velocity of the workpiece. This means that the prior lapping machines are ready for finish processing, but seldom possible to generate the desired curvature of the surfaces on the lens.
There are conventionally known many methods of generating the asymmetrical aspheric surface with high speed or high efficiency on the workpiece, using the numerical control (NC) processor. An example of the method of forming the asymmetrical aspheric surface on the workpiece is disclosed, for example, in Japanese Patent Laid-Open No. 309602/1999, in which a Z-axis table having a headstock thereon is kept against movement while on a processing operation. A workpiece mounted in a chuck on a work spindle is driven with a spindle motor to rotate on an axis of the work spindle, while a slider having a cutting tool thereon moves in and out in a Z-axis direction under NC control. Moreover, an X-axis table supporting the slider thereon travels in a reciprocating manner in the Z-axis direction. Thus, the slider and the X-axis table are allowed to reciprocate in synchronized relation not only with one another but also with the turning of the workpiece.
Another NC processor with onboard linear motor to get the slider reciprocating in Y-axis direction is disclosed, for example in Japanese Patent Laid-Open No. 2002-126907. With the prior NC processor as recited earlier, the linear motor forces the slider lying on a turner base to move in and out in a Z-axis direction, making it possible to cut the workpiece with high speed and high acceleration. To this end, the prior NC processor is comprised of a slide block provided thereon with a guide rail joined to the turner base in a way extending in the Z-axis direction perpendicular to an X-axis direction, a slider having a cutting tool to move back and forth along the guide rail with high speed and high acceleration, and a driving means to force the slider to move in and out along the linear guide rail, the driving means being composed of magnetic windings and field magnets allowed to move relatively to the magnetic windings, either of the magnetic windings and the field magnets being installed in the slider and the other in the slide block. There is provided the combination of a linear scale and a sensor to monitor a travel amount of the slider relative to the linear guide rail.
A prediction learning control system for a learning control to regulate the NC processor is moreover known as disclosed, for example in Japanese Patent Laid-Open No. 141004/1995. There is disclosed the learning control system, which has m inputs where target command vectors, and output vectors and state vectors of controlled object are input at current time in a way getting output vectors of controlled object represented with state space representation to follow target command vectors of the same pattern repeated with a period L, and p outputs where the control input vectors are output to the controlled object. The prediction learning control system recited above includes means for deriving a deviation vector from the combination of the target command vector and the output vector, means for storing a constant matrix for learning control, and means for determining a control input vector at current time to get a preselected evaluation function least.
Another method of forming the asymmetrical aspheric surface on the workpiece is disclosed in, for example Japanese Patent Laid-Open No. 2003-94201. With the method of forming the asymmetrical aspheric surface cited just earlier, a cutting tool is forced to move relatively into a workpiece rotated with a turning spindle head, thereby generating a desired surface on the workpiece. At the same time, the workpiece is moved in synchronized relation with the worked location in a radial direction of the turning spindle within a plane perpendicular to the rotational axis thereof, while the cutting tool is also moved depending on the worked location, carrying out the generation of the asymmetrical aspheric surface on the workpiece.
In the generation of any curved surface on the workpiece, most grinding or lapping operations always cost plenty of time for making any desired surface on the workpiece and, therefore, it remains a major challenge to produce any specific surface on the workpiece with short time. To cope with this, there have been developed the machining operations using the NC processor as stated just earlier to generate the asymmetrical aspheric surface with short time on the workpiece. With the prior machining operation using the NC processor, nevertheless, largely cutting hours needed for machining operation could not be achieved as the work spindle was kept against reciprocating movement in the Z-axis direction during cutting operation on the workpiece.
With the conventional processes for forming the desired curved surfaces on the workpiece recited earlier, the revolutions of the work spindle is determined and kept constant on the basis of the top acceleration in reciprocating movement of the cutting tool forced to move in and out by means of the Y-axis driving means. It is principal technical concept in the conventional generating process for the desired surface to keep continuously the revolutions of the work spindle on which the workpiece is held constant. According to most prior generating processes for the desired surface on the workpiece, that is to say, the rotating velocity of the work spindle is kept at a predetermined rpm throughout the cutting operation on the workpiece, which is determined in light of acceleration in the Y-axis reciprocating movement of the slider on which the cutting tool is clamped. This has been a major factor in substantial cycle time being needed for forming the desired surface on the workpiece. Momentum of the slider in Y-axis direction is less at the central area than at the peripheral area of the workpiece and yet the rotating velocity of the work spindle is set on the basis of the periphery of the workpiece without controlled to make any change. Getting the slider moving with the top acceleration in the Y-axis direction, thus, was realized at only the peripheral area of the workpiece and, therefore, no prior generating process for the curved surface on the workpiece could make the most of the high acceleration performance.
In the process for generating any curved surface on the workpiece including a thin lens such as spectacles, and so on, typically, the greater a travel amount of the slider in the Y-axis direction is the greatest at the time when any surface of the workpiece is generated into the curved surface by a circumferential fringe of the workpiece, and gets less as the cutting operation proceeds away from the circumferential fringe toward the center of the workpiece. Procedure to generate the curved surface on the workpiece such as lens and so on heads away from the circumferential fringe region larger in diameter towards the central region smaller in diameter of the workpiece. The movement of the slider in the X-axis direction carries out shifting the region where the desired curvature of surface is made on the workpiece from the radial outside inward. With the prior processes for generating any curved surface on the workpiece, moreover, the rotational frequency of the work spindle is selected in such a manner that the acceleration in Y-axis direction of the slider at the circumferential fringe of the workpiece is not more than the top acceleration that would be determined under the conditions of machine construction and performance. The work spindle is driven at the preselected number of revolution, which is kept constant throughout the generating process for the curved surface on the workpiece. The reason for keeping the rotational frequency of the work spindle constant throughout the generating procedure is that the prior process for generating the curved surface on the workpiece works under the learning control that adopts a time length as the period of learning control, necessitating keeping the time length per one revolution (360°) of the work spindle constant. In manufacture of, for example the thin lens such as spectacles in such a way proceeding the cutting operation away from the circumferential fringe toward the center of the workpiece, with using the prior generating process for the curved surface working on the basic principle of operation as stated just earlier, while the rotating frequency of the work spindle, or the time length per one revolution of the work spindle, is kept continuously constant throughout generating procedure for the curved surface, the travel amount in the Y axis of the slider on which the cutting tool is mounted gets less as the cutting operation proceeds away from the circumferential fringe toward the center of the lens. Thus, the acceleration in the Y axis would get less as the cutting operation proceeds away from the circumferential fringe toward the center of the lens.
With the curve generator in which a lead turner having a slider is onboard an X-axis table, on the other hand, the slider is so constructed as to move in the Y-axis direction with high speed and high acceleration during the curve-generating operation on the workpiece. With the prior generating process for the curved surface in which the Y-axis acceleration of the slider gets less as the cutting operation proceeds away from the fringe toward the center of the workpiece, accordingly, it could be said that the slider could not serve the high performance in the Y-axis movement of the slider to the full. Thus, the process for generating the curved surface on the workpiece still poses the major challenge about how to make the most of the Y-axis acceleration performance of the slider to shorten the time cycle it takes for the curve generation on the workpiece.